How Rooftop Solar Affects the GB Grid (and Your Bills)
Great Britain has over 15 GW of installed rooftop and ground-mounted solar PV. On a clear summer day, that capacity can generate enough electricity to power millions of homes — and it's all invisible to most of the people living under it.
Understanding when your panels generate, how they interact with the grid, and how to get the most value from them is increasingly worthwhile as electricity prices have risen and export tariff rates have improved.
How Much Solar Is Actually on the GB Grid?
The GB grid currently carries roughly 14–16 GW of solar PV capacity, split between:
- Rooftop PV (domestic and commercial): around 10–11 GW
- Solar farms: around 4–5 GW
On a clear day in June or July, these assets can collectively produce 10–13 GW between 10am and 2pm — equivalent to roughly 25–30% of national electricity demand during those hours. This is enough to push gas plants offline and suppress the wholesale electricity price.
You can see the current solar output in real time on the GB Generation Mix dashboard.
When Does Solar Generate Most?
Solar output follows a predictable pattern, but with significant seasonal variation:
| Month | Peak solar window | Typical peak output (GB-wide) |
|---|---|---|
| June–July | 10am–2pm | 10–13 GW |
| April–May | 10am–2pm | 7–10 GW |
| August–September | 10am–2pm | 6–9 GW |
| March | 10am–1pm | 4–6 GW |
| October–February | 11am–1pm | 1–3 GW |
For an individual domestic system (3–5 kWp typical), peak output on a summer day might be 2–3 kW, dropping to 0.5–1 kW on a clear winter day.
What Happens to Surplus Solar?
When your panels generate more than your home is consuming, the surplus flows back to the grid — a process called export. Three things happen to that exported electricity:
- It flows to neighbouring homes and businesses via the local distribution network — essentially your neighbours are consuming your solar
- It reduces the amount the grid needs to import from larger power stations
- You may be paid for it under the Smart Export Guarantee (SEG)
The SEG was introduced in 2020 and requires licensed electricity suppliers with 150,000+ customers to offer an export tariff. Rates vary significantly: from 4–5p/kWh for basic tariffs to 15–24p/kWh for premium tariffs from suppliers like Octopus (Outgoing) and OVO.
Smart Export Guarantee: Maximising Your Return
Not all SEG tariffs are created equal. The key distinction is between:
- Fixed rate SEG: You receive the same rate per kWh regardless of time of day — simple but misses opportunity
- Variable/time-of-use SEG: The rate changes based on grid conditions — you earn more when the grid needs your power most
Octopus Outgoing Agile is the standout variable-rate option: it pays you the half-hourly wholesale price for every unit you export. In practice this means:
- Midday summer solar earns less (when everyone else's solar is also flooding the grid, prices drop)
- Exporting on a cold, still, high-demand evening earns significantly more
If you have a home battery, this creates a powerful opportunity: store your midday solar, then export it during the evening peak when prices can be 5–10× higher than midday.
Self-Consumption: The Best Use of Your Solar
Exported electricity earns you money, but consuming solar yourself saves more — because you avoid buying electricity at retail rates (typically 25–35p/kWh) and only earn 4–24p/kWh on export.
The hierarchy of solar self-consumption:
- Immediate household use: Lights, appliances, whatever is running at the time
- Hot water: An immersion heater on a solar diverter (e.g. Eddi) can absorb surplus solar to heat your hot water tank — often the single highest-value use of surplus solar
- EV charging: If you're home and your car is plugged in, a solar-aware charger (e.g. Zappi in eco mode) will charge your car from the surplus
- Battery storage: Charge your home battery from solar and discharge it in the evening
How Solar Changes Your Relationship With the Grid
A home with solar shifts its grid import profile dramatically:
- Without solar: Importing grid power throughout the day and evening
- With solar (no battery): Near-zero import at midday, still importing in mornings and evenings
- With solar + battery: Potentially import very little or nothing on sunny days; morning and evening covered by battery discharge
This flattened import profile means solar households benefit disproportionately from time-of-use tariffs — particularly Octopus Agile, where the low midday prices (sometimes 5–10p/kWh) coincide with the period when solar-plus-battery homes are already self-sufficient. Your grid imports shift naturally to the cheapest windows.
Does Solar Affect Grid Carbon Intensity?
Yes — and significantly. When GB solar output is high (10–13 GW), gas plants are pushed to the margins or switched off entirely. This is why the carbon intensity of GB electricity is consistently lowest between 10am and 2pm on sunny spring and summer days.
If you're choosing when to run high-energy appliances and you don't have solar yourself, you're still benefiting from your neighbours' panels — the grid as a whole is cleaner during those hours.
The Case for Adding a Battery
A 5–10 kWh home battery (Tesla Powerwall, GivEnergy, Solis, etc.) transforms a solar installation:
- Capture midday surplus and use it after sunset
- Shift import to cheap overnight windows rather than expensive evening peaks
- Potential to arbitrage Octopus Agile — charge cheap overnight, export or self-consume at high-price periods
Payback periods have shortened as battery prices have fallen and electricity prices have risen. A 10 kWh battery paired with a 4 kWp solar system in southern England can now pay back in 6–9 years under favourable assumptions.
Summary
- GB solar contributes 25–30% of daytime electricity demand on sunny summer days
- Midday (10am–2pm) is the peak solar window — and the greenest time to use electricity
- Self-consumption beats export on most standard tariffs — prioritise using your own power
- Solar diverters and smart EV chargers can absorb surplus before it hits the export meter
- Adding a battery significantly improves economics, especially on variable-rate tariffs
- The Generation Mix shows live solar output for the whole GB grid